The present invention relates to a method and apparatus for regenerating a glass cleaning solution containing hydrofluoric acid, and a method and apparatus for cleaning silicate glass. In particular, the present invention relates to a method and apparatus for providing a glass surface requiring high cleanness as in a panel for a cathode-ray tube (CRT), for example. Furthermore, the present invention relates to a CRT equipped with the above-mentioned panel.
Hydrofluoric acid is used for etching a glass surface in producing glass for interior decoration, due to its property of dissolving silicate. Hydrofluoric acid also is used for cleaning glass, due to its property of corroding a glass surface. For example, when something remains adhering to the inner surface of a CRT glass panel for various displays and the like, a phosphor layer and the like formed on the panel are degraded, which results in a decrease in the performance of the CRT. Accordingly, a cleaning step using hydrofluoric acid is indispensable for producing a glass product requiring a very clean surface as in a CRT panel.
When cleaning a CRT panel, a glass cleaning solution containing hydrofluoric acid is circulated by a circulation apparatus connected to a panel cleaning tank so as to be used repeatedly for cleaning panels. However, when a cleaning solution is thus used repeatedly, substances adhering to glass surfaces and glass components are dissolved in the cleaning solution with an increasing number of cleaning cycles, which degrades the cleaning ability of the cleaning solution. Impurities such as dissolved glass components may adhere to a CRT panel to cause a panel defect. Under such circumstances, when cleaning a CRT panel, the cleaning solution is exchanged periodically. A used cleaning solution containing impurities is disposed of as industrial waste.
If a degraded cleaning ability is enhanced by supplying a cleaning solution with hydrofluoric acid, it becomes possible to extend the exchange cycle of a cleaning solution to some degree. However, simply supplying hydrofluoric acid will not overcome the problems related to reduction of industrial waste. Furthermore, supplying hydrofluoric acid cannot remove impurities in a cleaning solution to prevent a panel defect.
Among impurities in a cleaning solution, in particular, fluorosilicic acid (hexafluorosilicic acid; H2SiF6) generated by the reaction between silica (SiO2) in glass and hydrogen fluoride (HF) causes a serious problem. Fluorosilicic acid binds with various cations contained in the cleaning solution to form a fluorosilicate in a gel state. This gel with low flowability is transparent. Therefore, when it adheres to a glass surface, the gel is difficult to recognize visually when the glass surface is wet, which is likely to cause defects in a glass product.
As described in Japanese Publication for Opposition No. 46-15768, for example, it is possible to remove impurities in hydrofluoric acid by utilizing electrolysis. However, electrolysis is not suitable as a method for removing large amounts of impurities efficiently from an exhausted glass cleaning solution. Furthermore, although electrolysis is excellent as a method for removing a trace amount of cations from hydrofluoric acid, it is not effective as a method for removing fluorosilicic acid.
Cleaning a CRT panel is applied for reusing not only a newly molded panel but also a CRT panel on which a black matrix layer or a phosphor layer has been formed. By cleaning the panel with a cleaning solution containing hydrofluoric acid, the black matrix layer and the like are removed by etching together with glass on a panel surface layer. However, these layers contain substances (e.g., carbon in the black matrix layer) which are sparingly soluble in hydrofluoric acid. Therefore, in the cleaning step for reusing a CRT panel, minute insoluble substances are likely to be generated in a large amount in the cleaning solution. If the insoluble substances are left, the surface of a panel will be damaged; therefore, it is necessary to remove insoluble substances floating in the solution. As a method for removing insoluble substances, settling them in a settling tank and filtering them with a filter are considered.
The method for settling insoluble substances in a settling tank is effective when they are large insoluble particles; however, this method is not suitable for minute insoluble substances floating in a glass cleaning solution. Furthermore, according to the method for filtering insoluble substances with a filter, a filter is clogged more quickly if it has a high filtering ability, and a filter having a low filtering ability cannot remove insoluble substances.
As described above, conventionally, a method for regenerating efficiently a glass cleaning solution containing hydrofluoric acid is not known. Reduction of industrial waste has already become a social task, and it also has become a task for the purpose of continuing enterprise activities, in response to the implementation of International Standard for Environment (ISO14001). Under such a situation, an effective method for recovering the cleaning ability of a glass cleaning solution, other than that for exchanging a glass cleaning solution, has not been found. This is becoming a serious problem in the production of glass products.
In order to overcome the above-mentioned problem, an objective of the present invention is to provide an efficient method and apparatus for regenerating a glass cleaning solution containing hydrofluoric acid that is exhausted in a large amount, utilized for cleaning and treating silicate glass. In particular, an objective of the present invention is to provide an efficient method and apparatus for regenerating a glass cleaning solution containing floating insoluble substances and fluorosilicic acid.
Another objective of the present invention is to provide a method and apparatus for cleaning silicate glass with a regenerated cleaning solution.
Still another objective of the present invention is to provide a cathode-ray tube using a panel cleaned with a regenerated cleaning solution.
In order to achieve the above-mentioned objective, a method for regenerating a glass cleaning solution of the present invention is characterized by adding fluoride to a glass cleaning solution containing hydrofluoric acid after being used for cleaning a surface of silicate glass, allowing fluorosilicic acid in the glass cleaning solution to react with the fluoride to precipitate fluorosilicate, and removing the fluorosilicate from the glass cleaning solution.
According to the above-mentioned regeneration method, a large amount of glass cleaning solution can be regenerated efficiently. In particular, according to the above-mentioned method, hydrogen fluoride is generated in the precipitation of fluorosilicate, and the hydrogen fluoride contributes to the recovery of the cleaning ability of the glass cleaning solution. According to the regeneration method, since the glass cleaning solution can be regenerated by an industrial procedure, the exchange cycle of the glass cleaning solution can be extended.
In the regeneration method, it is preferable that fluoride is provided from at least one compound selected from the group consisting of lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, cesium fluoride, magnesium fluoride, strontium fluoride, barium fluoride, cobalt fluoride, manganese fluoride, copper fluoride, and ammonium fluoride. This is because fluorosilicic acid can be removed efficiently by using these fluorides.
Furthermore, in the above-mentioned regeneration method, it is preferable that hydrofluoric acid is supplied to the glass cleaning solution from which fluorosilicate has been removed. Furthermore, in the above-mentioned regeneration method, it is preferable that fluoride is added together with hydrofluoric acid. Hydrogen fluoride generated by the above-mentioned regeneration method does not cover all the hydrogen fluoride consumed, for example, in the generation of fluorosilicic acid. However, in the above-mentioned preferable example, the cleaning ability of the glass cleaning solution is sufficiently recovered, and the exchange cycle of the glass cleaning solution can be further extended.
Furthermore, in the above-mentioned regeneration method, it is preferable that the concentration of Si in the glass cleaning solution after being used for cleaning the surface of silicate glass is measured, and an amount of fluoride to be added is set at an amount or more required for all the Si to be changed to fluorosilicate. It is preferable that fluoride is added in an amount one to two times that required for all the Si to be changed to fluorosilicate. In the preferable example, as fluoride is added in an amount in accordance with the concentration of Si. Therefore, fluoride can be prevented from being added in such an excessive amount as to become an impurity, causing another problem and insufficient removal of fluorosilicic acid.
Furthermore, in order to achieve the above-mentioned objective, a method for cleaning silicate glass of the present invention is characterized in that the surface of silicate glass is cleaned with the glass cleaning solution regenerated by the above-mentioned regeneration method.
Furthermore, another method for cleaning silicate glass of the present invention is characterized in that a regeneration step of regenerating the glass cleaning solution regenerated in the above-mentioned regeneration method is conducted concurrently with a cleaning step of cleaning the surface of silicate glass with the glass cleaning solution obtained in the regeneration step, and the glass cleaning solution used in the cleaning step is regenerated in the regeneration step, whereby the glass cleaning solution is used while being regenerated.
According to the above-mentioned cleaning methods, the amount of industrial waste containing hydrofluoric acid can be reduced more compared with the prior art, and defects in a glass product caused by adhesion of fluorosilicate can be reduced.
According to the above-mentioned cleaning methods, it is preferable that the silicate glass is a panel for a cathode-ray tube. This is because a panel for a cathode-ray tube particularly requires a clean surface, and is cleaned with a glass cleaning solution containing a large amount of hydrofluoric acid.
In particular, the above-mentioned cleaning methods exhibit outstanding effects in the case where the silicate glass includes a panel on which surface at least one selected from the group consisting of a black matrix layer, a phosphor layer, and a metal back layer is present. Although substances insoluble in the glass cleaning solution are generated from each of the layers, these insoluble substances can be removed together with fluorosilicate according to the above-mentioned cleaning methods.
In order to achieve the above-mentioned objective, an apparatus for regenerating a glass cleaning solution of the present invention is characterized by including: a treatment tank for adding fluoride to a glass cleaning solution containing hydrofluoric acid after being used for cleaning a surface of silicate glass; and fluorosilicate separating means for removing fluorosilicate, precipitated by reaction between fluorosilicic acid in the glass cleaning solution and the fluoride, from the glass cleaning solution.
The above-mentioned regeneration apparatus enables a large amount of glass cleaning solution to be regenerated efficiently. The apparatus also ,enables insoluble substances floating in the glass cleaning solution to be removed together with fluorosilicate. Furthermore, in the above-mentioned apparatus, hydrogen fluoride is generated in precipitation of fluorosilicate, and the hydrogen fluoride contributes to the recovery of the cleaning ability of the glass cleaning solution. Thus, the above-mentioned regeneration apparatus enables a glass cleaning solution to be regenerated by an industrial procedure, so that the exchange cycle of the glass cleaning solution can be extended.
It is preferable that the above-mentioned regeneration apparatus further includes a settling tank for receiving the glass cleaning solution from a bottom of the treatment tank.
Furthermore, in the above-mentioned regeneration apparatus, the fluorosilicate separating means is not particularly limited, as long as it is an apparatus provided with a function of separating a solid from a liquid, and may be a filtering apparatus such as a filter. However, the fluorosilicate separating means preferably includes a drain cock provided on the bottom part of the treatment tank and/or the settling tank. This is because such a drain cock enables efficient separation and is preferable for continuous operation of the regeneration apparatus.
It is preferable that the above-mentioned regeneration apparatus further includes an adjustment tank for supplying hydrofluoric acid to the glass cleaning solution from which fluorosilicate has been removed. In the preferable example, the exchange cycle of the glass cleaning solution can be further extended. Furthermore, although not particularly limited, it is preferable that the above-mentioned adjustment tank is disposed so as to receive the glass cleaning solution from the vicinity of a surface of the solution in the above-mentioned settling tank (treatment tank in the absence of the settling tank).
In order to achieve the above-mentioned objective, an apparatus for cleaning silicate glass of the present invention is characterized by including the above-mentioned regeneration apparatus and a cleaning tank for receiving the glass cleaning solution from the regeneration apparatus and cleaning the surface of silicate glass with the glass cleaning solution.
Furthermore, another apparatus for cleaning silicate glass of the present invention is characterized by including the above-mentioned regeneration apparatus and a cleaning tank for receiving the glass cleaning solution from the regeneration apparatus and cleaning the surface of silicate glass with the glass cleaning solution, wherein the glass cleaning solution used in the cleaning tank is further regenerated in the regeneration apparatus, whereby the glass cleaning solution is used while being regenerated.
The above-mentioned regeneration apparatus enables the amount of industrial waste containing hydrofluoric acid to be reduced more compared with the prior art. The regeneration apparatus also enables the defects in a glass product caused by adhesion of fluorosilicate to be reduced.
Furthermore, according to the present invention, a cathode-ray tube is provided, which includes a panel for a cathode-ray tube cleaned by the above-mentioned cleaning method.